15. In a phase diagram, what term is used for a substance held at a temperature and pressure where solid, liquid, and gaseous states of a substance exist simultaneously?
Answer: B
The term used for a substance held at a temperature and pressure where solid, liquid, and gaseous states of a substance exist simultaneously is the triple point.
The triple point is a specific condition in a phase diagram where all three phases—solid, liquid, and gas—coexist in equilibrium.
A) critical temperature
Critical temperature refers to the highest temperature at which a substance can exist as a liquid. Above this temperature, no amount of pressure can liquefy the substance. This term does not describe the coexistence of multiple phases.
B) triple point
The triple point is the correct answer, as it specifically defines the unique set of conditions (temperature and pressure) at which a substance can exist simultaneously in all three phases: solid, liquid, and gas.
C) absolute zero
Absolute zero is the theoretical temperature at which a system reaches its lowest possible energy state, corresponding to 0 Kelvin. At this point, molecular motion ceases, and it does not refer to the coexistence of phases.
D) critical point
The critical point represents the end of the phase boundary between liquid and gas, beyond which distinct liquid and gas phases do not exist. It does not indicate the simultaneous presence of solid, liquid, and gas phases.
Conclusion
The triple point is the only term that accurately describes the unique conditions under which solid, liquid, and gaseous states of a substance coexist. All other options either refer to different concepts or conditions that do not encompass the simultaneous existence of all three phases. Thus, the triple point is definitively the correct choice.